Control valves and chokes, as well as other types of valves, are frequently used in the oil and gas industry. There are numerous designs of actuators which have been applied to such valves for remote operation. Some actuator designs also incorporate an emergency shutdown feature. The shutdown components can be actuated electrically or hydraulically. Some applications do not have an adjacent available electrical supply or an auxiliary hydraulic system for actuation of the shutdown sequence for the valve. Shutdown systems in such environments have relied on line pressure changes for the shutdown trigger.
U.S. Pat. Nos. 1,466,945; 1,784,094; 2,327,980; 2,930,571; 3,168,841; 3,512,421; 3,518,891; 3,640,140; 3,889,924; 3,921,264; 3,955,792; 4,082,247; 4,090,589; 4,130,030; 4,189,950; 4,256,065; 4,273,307; 4,323,221; 4,436,278; 4,651,969; 4,741,508; 4,749,004; 4,771,643; 4,920,811; 4,920,816; 5,195,721; 5,497,672; EP 0 512 139 A1; WO 80/00483; WO 96/04494; 785,188; 1,141,082; 1,340,570; 1,186,767; GB 2 243 669 A; GB 2 283 061 A; literature on Dresser Model "SRM" Safety Release Manual Actuator; Baker Oil Tools Maintenance and Operating Instructions for Model "CSWC" Coil Spring, High Thrust Wire-Cutting Actuator; and Baker Oil Tools literature on Pneumatic "Piston" Actuator, show generally the variety of types of actuators that have been employed in the past and some executions of shutdown sequences incorporated in those designs. U.S. Pat. No. 5,261,446, issued to the inventor of the present invention, Gerald S. Baker, illustrated a shutdown system involving actuation of a torsion spring which worked with the main closure spring in the actuator to accomplish emergency shutdown. The tripping system for the shutdown modes is illustrated in FIGS. 8-10 of this patent. This system employed a piston-sensing line pressure and a series of complex linkages upon which various spring-loaded plungers would act so as to allow actuation of the tripping mechanism if the sensed line pressure exceeded a high-pressure setpoint or became less than a low-pressure setpoint. One of the problems with the design illustrated in U.S. Pat. No. 5,261,446 is that when the setpoints for tripping upon sensed high or low pressure were set fairly close together, the mechanism was incapable of being reset. As the line pressure rose in an attempt to reset the mechanism in U.S. Pat. No. 5,261,446, the high pressure setpoint trip would also be actuated, precluding resetting of the device. This generally became a problem if the difference between the high- and low-pressure trip points was less than about 400 psi. This problem occurred due to the linkages employed, which had interdependent movements, thus making the reset procedure difficult, if not impossible, for low-differential setpoints for high- and low-pressure trips.
Accordingly, what is needed is an actuator for a valve which can be easily reset and which can be tripped without the need for auxiliary electrical or hydraulic systems. Additionally, an objective of the present invention is to provide independently operating high- and low-pressure trip assemblies where the operation of one trip assembly is independent of the operation of the other. Another objective is to provide alternative shutdown techniques such as, for example, in the event of a fire in the area of a valve mounted to the actuator. Another objective is to provide a compact design to accommodate installations where space is at a premium. Yet another objective is to provide a variety of ways for manually tripping the valve to the closed position. Yet another objective is to retain the valve in the trip position until manually reset. Those and other features of the invention will become more apparent to those skilled in the art from a review of the preferred embodiment and an alternative embodiment described below.